cDNA sequencing of nuclear lamins A and C reveals primary and secondary structural homology to intermediate filament proteins

Internuclear exchange of an inner nuclear membrane protein (p55) in heterokaryons: in vivo evidence for the interaction of p55 with the nuclear lamina

The movement between nuclei of an integral protein of the inner nuclear membrane has been studied in rat/mouse and rat/hamster heterokaryons. This protein, p55, was found to equilibrate between nuclei over a period of approximately 6 h in the absence of new protein synthesis. When rat/mouse heterokaryons were constructed using an undifferentiated murine embryonal carcinoma (P19), which lacks lamins A and C, no accumulation of p55 in the mouse cell nucleus was observed. However, P19 nuclei could be rendered competent to accumulate p55 by transfecting the parent cells with human lamin A before cell fusion, supporting the notion that p55 may interact with the nuclear lamina. Since p55 does not appear to be able to dissociate from the nuclear membrane, it is concluded that this exchange between nuclei does not occur in the aqueous phase and instead is probably membrane mediated. It is proposed that this protein may be free to move between the inner and outer nuclear membranes via the continuities at the nuclear pore complexes and that transfer between nuclei occurs via lateral diffusion through the peripheral ER, which appears to form a single continuous membrane system in these heterokaryons. One implication of these observations is that accumulation of at least some integral proteins in the inner nuclear membrane may be mediated by interactions with other nuclear components and may not require a single defined targeting sequence.

Deletions in epidermal keratins leading to alterations in filament organization in vivo and in intermediate filament assembly in vitro

To investigate the sequences important for assembly of keratins into 10-nm filaments, we used a combined approach of (a) transfection of mutant keratin cDNAs into epithelial cells in vivo, and (b) in vitro assembly of mutant and wild-type keratins. Keratin K14 mutants missing the nonhelical carboxy- and amino-terminal domains not only integrated without perturbation into endogenous keratin filament networks in vivo, but they also formed 10-nm filaments with K5 in vitro. Surprisingly, keratin mutants missing the highly conserved L L E G E sequence, common to all intermediate filament proteins and found at the carboxy end of the alpha-helical rod domain, also assembled into filaments with only a somewhat reduced efficiency. Even a carboxy K14 mutant missing approximately 10% of the rod assembled into filaments, although in this case filaments aggregated significantly. Despite the ability of these mutants to form filaments in vitro, they often perturbed keratin filament organization in vivo. In contrast, small truncations in the amino-terminal end of the rod domain more severely disrupted the filament assembly process in vitro as well as in vivo, and in particular restricted elongation. For both carboxy and amino rod deletions, the more extensive the deletion, the more severe the phenotype. Surprisingly, while elongation could be almost quantitatively blocked with large mutations, tetramer formation and higher ordered lateral interactions still occurred. Collectively, our in vitro data (a) provide a molecular basis for the dominance of our mutants in vivo, (b) offer new insights as to why different mutants may generate different phenotypes in vivo, and (c) delineate the limit sequences necessary for K14 to both incorporate properly into a preexisting keratin filament network in vivo and assemble efficiently into 10-nm keratin filaments in vitro.

Cellular intermediate filament networks and their derangement in alcoholic hepatitis

Intermediate filaments are major components of most eukaryotic cells that form from the polymerization of protein subunits that are expressed in tissue and development specific fashions. The interactions of intermediate filaments with a myriad of other cellular proteins and structures give rise to a complex overall cellular architecture that is likely responsible for cellular well-being. The mature 10-nm filaments are relatively stable cellular structures, but the intermediate filaments undergo major morphological and biochemical changes, especially during mitosis, differentiation, and in response to certain drugs. Evidence exists that hepatocyte intermediate filaments (keratin filaments) are deranged in alcoholic hepatitis, an inflammatory liver disease of alcoholics and heavy spree drinkers. The classical and characteristic pathological hepatocyte inclusion bodies of alcoholic hepatitis, Mallory bodies, are composed in part of normal keratins that likely derive from the pre-existing hepatocyte intermediate filament network. It is unclear if intermediate filament network derangement in alcoholic hepatitis is directly caused by the actions of ethanol or its metabolites on intermediate filaments or their associated structures, or whether alcohol causes a cellular insult or injury elsewhere and a subsequent response (e.g., immune) causes intermediate filament network derangement. The precise mechanisms responsible for intermediate filament derangement remain to be elucidated; however, experimental data exist that support and refute several hypotheses. Hopefully, further studies will help determine a better overall understanding of the abnormalities of intermediate filaments and their relationship to the pathophysiology of alcoholic hepatitis and other diseases.

A lamin-independent pathway for nuclear envelope assembly

The nuclear envelope is composed of membranes, nuclear pores, and a nuclear lamina. Using a cell-free nuclear assembly extract derived from Xenopus eggs, we have investigated how these three components interact during nuclear assembly. We find that the Xenopus embryonic lamin protein LIII cannot bind directly to chromatin or membranes when each is present alone, but is readily incorporated into nuclei when both of the components are present together in an assembly extract. We find that depleting lamin LIII from an extract does not prevent formation of an envelope consisting of membranes and nuclear pores. However, these lamin-depleted envelopes are extremely fragile and fail to grow beyond a limited extent. This suggests that lamin assembly is not required during the initial steps of nuclear envelope formation, but is required for later growth and for maintaining the structural integrity of the envelope. We also present results showing that lamins may only be incorporated into nuclei after DNA has been encapsulated within an envelope and nuclear transport has been activated. With respect to nuclear function, our results show that the presence of a nuclear lamina is required for DNA synthesis to occur within assembled nuclei.

Human P450scc gene transcription is induced by cyclic AMP and repressed by 12-O-tetradecanoylphorbol-13-acetate and A23187 through independent cis elements

Long-term regulation of mammalian steroid hormone synthesis occurs principally by transcriptional regulation of the gene for the rate-limiting cholesterol side-chain cleavage enzyme P450scc. Adrenal steroidogenesis is regulated primarily by two hormones: adrenocorticotropin, which works via cyclic AMP (cAMP) and protein kinase A, and angiotensin II, which works via Ca2+ and protein kinase C. Forskolin and 8-bromo-cAMP stimulated, while prolonged treatment with a phorbol ester (12-O-tetradecanoylphorbol-13-acetate [TPA]) and a calcium ionophore (A23187) additively suppressed accumulation of endogenous P450scc mRNA in transformed murine adrenal Y1 cells. In Y1 cells transfected with 2,327 base pairs of the human P450scc promoter fused to the bacterial gene for chloramphenicol acetyltransferase (CAT), forskolin increased CAT activity 900% while combined TPA plus A23187 reduced CAT activity to 15% of the control level. Forskolin induced the P450scc promoter as rapidly as a promoter containing two cAMP-responsive elements fused to a simian virus 40 promoter, a system known to respond directly to cAMP. Basal expression was increased by sequences between -89 and -152 and was increased further by sequences between -605 and -2327. This upstream region also conferred inducibility by cAMP. TPA plus A23187 transiently increased CAT activity before repressing it, reflecting the complex actions of angiotensin II in vivo. Repression by prolonged treatment with TPA plus A23187 was mediated by multiple elements between -89 and -343. Induction of CAT activity by forskolin was not diminished by treatment with TPA plus A23187, nor were the regions of the promoter responsible for regulation by the two pathways coisolated. Thus, the human gene for P450scc is repressed by TPA plus A23187 by mechanisms and sequences independent of those that mediate induction by cAMP.

Transformation of the amphibian oocyte into the egg: structural and biochemical events

Amphibian oocytes, arrested in prophase I, are stimulated to progress to metaphase II by progesterone. This process is referred to as meiotic maturation and transforms the oocyte, which cannot support the early events of embryogenesis, into the egg, which can. Meiotic maturation entails global reorganization of cell ultrastructure: In the cell cortex, the plasma membrane flattens and the cortical granules undergo redistribution. In the cell periphery, the annulate lamellae disassemble and the mitochondria become dispersed. In the cell interior, the germinal vesicle becomes disassembled and the meiotic spindles form. Marked changes in the cytoskeleton and mRNA distribution also occur throughout the cell. All of these events are temporally correlated with intracellular signalling events: Fluctuations in cAMP levels, changes in pH, phosphorylation and dephosphorylation, and ion flux changes. Evidence suggests that specific intracellular signals are responsible for specific reorganizations of ultrastructure and mRNA distribution.

Assembly properties of dominant and recessive mutations in the small mouse neurofilament (NF-L) subunit

We have generated a set of amino- and carboxy-terminal deletions of the NF-L neurofilament gene and determined the assembly properties of the encoded subunits after coexpression with vimentin or wild-type NF-L. NF-L molecules missing greater than 30% (31 amino acids of the head) or 90% (128 amino acids of the tail) failed to incorporate into intermediate filament networks. Carboxy-terminal deletions into the rod domain yield dominant mutants that disrupt arrays assembled from wild-type subunits, even when present at levels of approximately 2% of the wild-type subunits. Even mutants retaining 55% of the tail (61 amino acids) disrupt normal arrays when accumulated above approximately 10% of wild-type subunits. Since deletion of greater than 90% of the head domain produces "recessive" assembly incompetent subunits that do not affect wild-type filament arrays, whereas smaller deletions yield efficient network disruption, we conclude that some sequence(s) in the head domain (within residues 31-87) are required for the earliest steps in filament assembly. Insertional mutagenesis in the nonhelical spacer region within the rod domain reveals that as many as eight additional amino acids can be tolerated without disrupting assembly competence.

Antibodies to nuclear lamin C in chronic hepatitis delta virus infection

Sera of patients with chronic hepatitis delta virus infection stained the nuclear periphery in indirect immunofluorescence. Using proteins of isolated nuclei, isolated nuclear matrices, the nuclear pore complex-lamina fraction and purified lamins A and C as antigen source in immunoblotting experiments, nuclear lamin C was identified as the reactive antigen. Most sera tested (8 of 10) recognized nuclear lamin C exclusively, but not the nuclear lamins A and B. Antibodies reacting with both nuclear lamins A and C, which share extensive sequence homologies, have been reported to occur in autoimmune hepatitis and primary biliary cirrhosis. The present findings suggest that the novel autoantibody associated with chronic hepatitis delta virus infection recognizes an epitope localized in the short carboxyterminal region of nuclear lamin C.

The lamin B receptor of the nuclear envelope inner membrane: a polytopic protein with eight potential transmembrane domains

The lamin B receptor is a previously identified integral membrane protein in the nuclear envelope of turkey erythrocytes that associates with the nuclear intermediate filament protein lamin B (Worman, H. J., J. Yuan, G. Blobel, and S. D. Georgatos. 1988. Proc. Natl. Acad. Sci. USA. 85:8531-8534). In the present report, we use cell fractionation and antibodies against the lamin B receptor to localize it to an 8-M urea-extracted membrane fraction of chicken liver nuclei, supporting an inner nuclear membrane localization. We deduced the amino acid sequence of the chicken lamin B receptor from overlapping clones obtained by screening cDNA libraries with a probe generated by the polymerase chain reaction with primers based on the partial protein sequence of the isolated protein. The mature lamin B receptor has a calculated molecular mass of 73,375 D and eight segments of hydrophobic amino acids that could function as transmembrane domains as determined by hydropathy analysis. Preceding the first putative transmembrane segment is a highly charged 204-residue-long amino terminal region that contains two consensus sites for phosphorylation by protein kinase A. Since the lamin B receptor has been shown to be phosphorylated by protein kinase A in vitro and in vivo and this phosphorylation affects lamin B binding (Applebaum, J., G. Blobel, and S. D. Georgatos. 1990. J. Biol. Chem. 265:4181-4185), it is likely that this amino terminal region faces the nucleoplasm. The amino terminal region also contains three DNA-binding motifs that are found in gene regulatory proteins and histones, suggesting that the lamin B receptor may additionally play a role in gene regulation and/or chromatin organization.

Characterization of a second highly conserved B-type lamin present in cells previously thought to contain only a single B-type lamin

Previous analyses of the nuclear lamina of mammalian cells have revealed three major protein components (lamins A, B and C) that have been identified by protein sequence homology as members of the intermediate filament (IF) protein family. It has been claimed that mammalian cells contain either all three lamins or lamin B alone. Using monoclonal antibodies specific for B-type lamins and cDNA cloning we identified a second major mammalian B-type lamin (murine lamin B2), thus showing that lamin composition in mammals is more complex than previously thought. Lamin B2 is coexpressed with lamin B1 (formerly termed lamin B) in all somatic cells and mammalian species that we analysed, including a variety of cells currently believed to contain only a single lamin. This suggests that two B-type lamins are necessary to form a functional lamina in mammalian somatic cells. By cDNA cloning we found that Xenopus laevis lamin LII is the amphibian homolog of mammalian lamin B2. Lamin expression during embryogenesis of amphibians and mammals shows striking similarities. The first lamins expressed in the early embryo are the two B-type lamins, while A-type lamins are only detected much later in development. These findings indicate that the genomic differentiation into two B-type lamins occurred early in vertebrate evolution and has been maintained in both their primary structure and pattern of expression.

Identification and characterization of autoantibodies against the nuclear envelope lamin B receptor from patients with primary biliary cirrhosis

We have identified autoantibodies from two patients with primary biliary cirrhosis (PBC) that recognize the nuclear envelope of mammalian cells on indirect immunofluorescence microscopy. These antibodies bind to a 58-kD integral membrane protein (p58) of the turkey erythrocyte nuclear envelope, which has been previously identified as a membrane receptor for lamin B (Worman, H. J., J. Yuan, G. Blobel, and S. D. Georgatos. 1988. Proc. Natl. Acad. Sci. USA. 85:8531). The antibodies also bind to a 61-kD integral membrane protein (p61) of the rat liver nuclear envelope. Affinity-purified antibodies eluted from turkey p58 bind to rat p61, showing that the two proteins share an epitope(s) and that p61 is likely the rat liver lamin B receptor. In human nuclear envelopes, the antigen recognized has an apparent molecular mass close to that of avian protein. These findings, along with the previous discovery of autoantibodies against an integral membrane glycoprotein (gp210) of the nuclear pore membrane in patients with PBC, suggest that antibodies against integral membrane proteins of the nuclear envelope are characteristic of a subset of patients with PBC.

Lamins A and C bind and assemble at the surface of mitotic chromosomes

To study a possible interaction of nuclear lamins with chromatin, we examined assembly of lamins A and C at mitotic chromosome surfaces in vitro. When a postmicrosomal supernatant of metaphase CHO cells containing disassembled lamins A and C is incubated with chromosomes isolated from mitotic Chinese hamster ovary cells, lamins A and C undergo dephosphorylation and uniformly coat the chromosome surfaces. Furthermore, when purified rat liver lamins A and C are dialyzed with mitotic chromosomes into a buffer of physiological ionic strength and pH, lamins A and C coat chromosomes in a similar fashion. In both cases a lamin-containing supramolecular structure is formed that remains intact when the chromatin is removed by digestion with micrococcal nuclease and extraction with 0.5 M KCl. Lamins associate with chromosomes at concentrations approximately eightfold lower than the critical concentration at which they self-assemble into insoluble structures in the absence of chromosomes, indicating that chromosome surfaces contain binding sites that promote lamin assembly. These binding sites are destroyed by brief treatment of chromosomes with trypsin or micrococcal nuclease. Together, these data suggest the existence of a specific lamin-chromatin interaction in cells that may be important for nuclear envelope reassembly and interphase chromosome structure.

Expression and characterization of human lamin C

We have expressed human lamin C cDNA in E. coli using a modification of the pLcII vector system. Protein produced in this way had seven additional amino acids at its N-terminus, but retained key lamin structural and assembly properties. The modified vector we produced may prove useful when difficulties are encountered in removal of the cII fusion peptide by factor X cleavage in the pLcII system. Shadowed preparations of expressed lamin C showed the presence of 50-nm rod-like particles that closely resembled those observed for native material. Isolated molecules had two globular domains at one end, indicating that they were constructed from two parallel polypeptide chains. The expressed material also formed paracrystals with a characteristic 22.5 nm axial repeat, indicating that its assembly properties had also been retained. We also used site-specific mutagenesis to engineer a lamin fragment that lacked the C-terminal non-helical domain of the molecule. This material formed paracrystals similar to those obtained with the intact molecule, indicating that the large C-terminal non-helical domain did not contain information vital for lamin assembly.

Elucidating the early stages of keratin filament assembly

Because of extraordinarily tight coiled-coil associations of type I and type II keratins, the composition and structure of keratin subunits has been difficult to determine. We report here the use of novel genetic and biochemical methods to explore the early stages of keratin filament assembly. Using bacterially expressed humans K5 and K14, we show that remarkably, these keratins behave as 1:1 complexes even in 9 M urea and in the presence of a reducing agent. Gel filtration chromatography and chemical cross-linking were used to identify heterodimers and heterotetramers as the most stable building blocks of keratin filament assembly. EM suggested that the dimer consists of a coiled-coil of K5 and K14 aligned in register and in parallel fashion, and the tetramer consists of two dimers in antiparallel fashion, without polarity. In 4 M urea, both end-to-end and lateral packing of tetramers occurred, leading to a variety of larger heteromeric complexes. The coexistence of multiple, higher-ordered associations under strongly denaturing conditions suggests that there may not be a serial sequence of events leading to the assembly of keratin intermediate filaments, but rather a number of associations may take place in parallel.

Pan: a transcriptional regulator that binds chymotrypsin, insulin, and AP-4 enhancer motifs

Two cDNAs were cloned that encode proteins (Pan-1 and Pan-2, with predicted molecular masses of 67.4 and 67.7 kD, respectively) that bind to a conserved transcriptional regulatory element present in pancreatic exocrine genes. The cDNAs are virtually identical in the region that encodes the amino-terminal 525 amino acids; they differ only by a 3-nucleotide insertion that results in the addition of a single amino acid. The cDNAs, however, code for related, but divergent, carboxy-terminal regions. Expression of the cloned cDNAs in HeLa cells activates expression of a cotransfected chimeric gene containing multimers of the pancreatic conserved element linked to a minimal promoter. Pan-1 and Pan-2 bind to the consensus sequence CAC/GCTGT/C, the CACCTGTC motif, which is present in transcriptional control elements of several genes. Both Pan-1 and Pan-2 bind to related motifs present in the rat insulin enhancer as well as a DNA segment containing the SV40 AP-4 element. The Pan-1 and Pan-2 cDNAs display extensive sequence identity with partial cDNA clones encoding E12 and E47, which bind to the immunoglobulin kappa E2 cis-active element, and may be derived from the same gene. These factors belong to a set of genetically distinct molecules, including AP-4 and MLTF, that bind to the CACCTGTC motif or related sequences. The Pan/E12,E47 proteins also show structural similarity with the Drosophila daughterless protein, MyoD, Myogenin, and Myf-5.

In vitro posttranslational modification of lamin B cloned from a human T-cell line

Autoimmune diseases are characterized by spontaneously occurring autoantibodies which have proven to be useful reagents for the characterization of specific nuclear proteins. Using a monoclonal autoantibody (72B9) derived from a murine lupus strain, we have cloned a cDNA from the human T-cell line MOLT-4, which encodes nuclear lamin B. The identity of the encoded protein as lamin B was established by both biochemical and immunological criteria. Inspection of the deduced amino acid sequence of lamin B revealed the presence in coil 1B of the alpha-helical domain of a leucine heptad repeat region. Analysis of mRNA in HL60 and MOLT-4 cells, which express only lamin B, or HeLa cells, which express all three major lamins (A, B, and C), together with the comigration of in vitro-translated product with isolated HeLa cell lamin B by two-dimensional gel electrophoresis, suggests that a single lamin B is expressed in mammalian somatic cells. In vitro translation with the cDNA clone revealed an EDTA-sensitive posttranslational modification which resulted in an increase in the apparent molecular weight to that equivalent to the native in vivo-synthesized lamin B protein. This in vitro modification included incorporation of a product of mevalonolactone and required an intact carboxy terminus.

In vitro posttranslational modification of lamin B cloned from a human T-cell line

Autoimmune diseases are characterized by spontaneously occurring autoantibodies which have proven to be useful reagents for the characterization of specific nuclear proteins. Using a monoclonal autoantibody (72B9) derived from a murine lupus strain, we have cloned a cDNA from the human T-cell line MOLT-4, which encodes nuclear lamin B. The identity of the encoded protein as lamin B was established by both biochemical and immunological criteria. Inspection of the deduced amino acid sequence of lamin B revealed the presence in coil 1B of the alpha-helical domain of a leucine heptad repeat region. Analysis of mRNA in HL60 and MOLT-4 cells, which express only lamin B, or HeLa cells, which express all three major lamins (A, B, and C), together with the comigration of in vitro-translated product with isolated HeLa cell lamin B by two-dimensional gel electrophoresis, suggests that a single lamin B is expressed in mammalian somatic cells. In vitro translation with the cDNA clone revealed an EDTA-sensitive posttranslational modification which resulted in an increase in the apparent molecular weight to that equivalent to the native in vivo-synthesized lamin B protein. This in vitro modification included incorporation of a product of mevalonolactone and required an intact carboxy terminus.

Isoprenylation is required for the processing of the lamin A precursor

The nuclear lamina proteins, prelamin A, lamin B, and a 70-kD lamina-associated protein, are posttranslationally modified by a metabolite derived from mevalonate. This modification can be inhibited by treatment with (3-R,S)-3-fluoromevalonate, demonstrating that it is isoprenoid in nature. We have examined the association between isoprenoid metabolism and processing of the lamin A precursor in human and hamster cells. Inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase by mevinolin (lovastatin) specifically depletes endogenous isoprenoid pools and inhibits the conversion of prelamin A to lamin A. Prelamin A processing is also blocked by mevalonate starvation of Mev-1, a CHO cell line auxotrophic for mevalonate. Moreover, inhibition of prelamin A processing by mevinolin treatment is rapidly reversed by the addition of exogenous mevalonate. Processing of prelamin A is, therefore, dependent on isoprenoid metabolism. Analysis of the conversion of prelamin A to lamin A by two independent methods, immunoprecipitation and two-dimensional nonequilibrium pH gel electrophoresis, demonstrates that a precursor-product relationship exists between prelamin A and lamin A. Analysis of R,S-[5-3H(N)]mevalonate-labeled cells shows that the rate of turnover of the isoprenoid group from prelamin A is comparable to the rate of conversion of prelamin A to lamin A. These results suggest that during the proteolytic maturation of prelamin A, the isoprenylated moiety is lost. A significant difference between prelamin A processing, and that of p21ras and the B-type lamins that undergo isoprenylation-dependent proteolytic maturation, is that the mature form of lamin A is no longer isoprenylated.

Protein chemical analysis of purified murine lamin B identifies two distinct polypeptides B1 and B2

Lamin B purified from murine EAT cells was characterized by partial protein sequences. Contrary to the current view that mammals express only a single lamin B polypeptide corresponding to a characterized murine cDNA clone, our analysis documents two distinct B lamins. One protein follows the established cDNA sequence while the other identifies a novel murine lamin B. Comparison with the two chicken lamin B sequences established by cDNA cloning identifies the first murine lamin B sequence as a B1 type and the second as a B2 type. We conclude that mammals express two distinct lamin B forms as established by others for chicken.

Cloning and nucleotide sequence of cDNA for Ki antigen, a highly conserved nuclear protein detected with sera from patients with systemic lupus erythematosus

Patients with systemic lupus erythematosus (SLE) produce autoantibodies against a variety of nuclear antigens including Ki antigen. Although anti-Ki autoantibodies were found in a significant number of SLE patients, the nature of Ki antigen is poorly characterized. By using anti-Ki serum as a probe we have cloned a bovine cDNA directing the synthesis in Escherichia coli of a polypeptide immunologically indistinguishable from the authentic Ki antigen. A homologous human cDNA was also cloned and its nucleotide sequence predicted the entire primary structure of a novel nuclear protein with a molecular weight of 29 508 and with highly hydrophilic and weakly acidic character. The gene is highly conserved not only in the coding region but also in the 3'-untranslated region. The bacterially produced Ki antigen would be valuable for diagnosis of SLE.

Modification of nuclear lamin proteins by a mevalonic acid derivative occurs in reticulocyte lysates and requires the cysteine residue of the C-terminal CXXM motif

The C-terminus of nuclear lamins (CXXM) resembles a C-terminal motif (the CAAX box) of fungal mating factors and ras-related proteins. The CAAX box is subject to different types of post-translational modifications, including proteolytic processing, isoprenylation and carboxyl methylation. By peptide mapping we show that both chicken lamins A and B2 are processed proteolytically in vivo. However, whereas the entire CXXM motif is cleaved from lamin A, at most three C-terminal amino acids are removed from lamin B2. Following translation of cDNA-derived RNAs in reticulocyte lysates, lamin proteins specifically incorporate a derivative of [14C]mevalonic acid (MV), i.e. the precursor of a putative isoprenoid modification. Remarkably, no MV is incorporated into lamin B2 translated from a mutant cDNA encoding alanine instead of cysteine in the C-terminal CXXM motif. These results implicate this particular cysteine residue as the target for modification of lamin proteins by an isoprenoid MV derivative, and they indicate that isoprenylation is amenable to studies in cell-free systems. Moreover, our observations suggest that C-terminal processing of newly synthesized nuclear lamins is a multi-step process highly reminiscent of the pathway elaborated recently for ras-related proteins.

Specific disruption of intermediate filaments and the nuclear lamina by the 19-kDa product of the adenovirus E1B oncogene

The 19-kDa protein encoded within the adenovirus E1B gene is essential for transformation by adenovirus and for proper regulation of viral early gene transcription. In order to investigate the biological function of the 19-kDa E1B protein, vectors were constructed to produce the 19-kDa protein in mammalian cells under the direction of heterologous promoters. Surprisingly, during transient expression, the E1B 19-kDa protein specifically associated with and disrupted the organization of intermediate filaments and the nuclear lamina, without disturbing the organization of other cytoskeletal networks. These results directly demonstrate an effect of a viral transforming protein on the cytoskeleton and suggest a role for intermediate filaments and the nuclear lamina in modulation of viral gene expression and the process of oncogenic transformation.

The nucleoskeleton and the topology of transcription

Transcription is conventionally believed to occur by passage of a mobile polymerase along a fixed template. Evidence for this model is derived almost entirely from material prepared using hypotonic salt concentrations. Studies on subnuclear structures isolated using hypertonic conditions, and more recently using conditions closer to the physiological, suggest an alternative. Transcription occurs as the template moves past a polymerase attached to a nucleoskeleton; this skeleton is the active site of transcription. Evidence for the two models is summarised. Much of it is consistent with the polymerase being attached and not freely diffusible. Some consequences of such a model are discussed.

Glycosylated forms of nuclear lamins

Chromatin and pore complex-lamina preparations were obtained from pig and chicken tissues, and their proteins were analysed by mono- and bidimensional electrophoresis. A glycosylated form of lamin A, recognized by concanavalin A, was shown to be present in at least 3 of the tissues examined. Glycosylation is suggested to be a further postsynthetic modification, besides phosphorylation and methylation, which can modify the properties of lamins.

Maturation of nuclear lamin A involves a specific carboxy-terminal trimming, which removes the polyisoprenylation site from the precursor; implications for the structure of the nuclear lamina

Lamin A, a nuclear lamina protein of differentiated cells, is synthesized as a precursor of the mature molecule. Protein sequencing of the carboxy-terminal 14 kDa fragment shows a lack of the last 18 residues predicted by cDNA sequencing. The carboxy-terminal proteolytic maturation explains previous biochemical results including the loss of the polyisoprenylation site now located to the CXXM motif at the end of the chain. This view and earlier results on lamin B predict multiple post-translational modifications shared by lamins A and B. While retained by lamin B, which is present in all cells, they are lost by maturation from lamin A, which probably acts only as an additional lamina constituent in differentiated cells.

The conserved carboxy-terminal cysteine of nuclear lamins is essential for lamin association with the nuclear envelope

We have analyzed the interaction of soluble nuclear lamins with the nuclear envelope by microinjection of normal and mutated lamins into the cytoplasm of Xenopus laevis oocytes. Our results demonstrate that the conserved cysteine of the carboxy-terminal tetrapeptide Cys Ala/Ser Ile Met of lamins is essential for their association with the nuclear envelope. Removal of this sequence or replacement of the cysteine by serine resulted in Xenopus lamin L1 remaining in a soluble, non-envelope-associated state within the nucleus. Similar mutations of Xenopus lamin A resulted in only partial reduction of nuclear envelope association, indicating that lamin A contains additional signals that can partially compensate for the lack of the cysteine. Mammalian lamin C lacks this tetrapeptide and is not associated with the nuclear envelope in our experimental system. Cloning of the tetrapeptide Cys Ala Ile Met to the carboxy terminus of human lamin C resulted in lamin being found in a nuclear envelope-associated form in oocytes. Mutations at the amino terminus and in the alpha-helical region of lamin L1 revealed that the carboxy terminus mediates the association of lamins with the nuclear envelope; however, this alone is insufficient for maintenance of a stable association with the nuclear envelope.

Interactions and structure of the nuclear pore complex revealed by cryo-electron microscopy

Nuclear pore complexes (NPCs) play a central role in mediating nucleocytoplasmic transport and exchange processes in eukaryotic cells. The arrangement and interactions of NPCs within amphibian nuclear envelopes have been studied using cryo-electron microscopy of unfixed and frozen hydrated specimens. The nuclear lamina in Necturus forms an orthogonal network with crossover distances which vary between 1,600 and 4,000 A and which may be related to the basic filament repeat of lamins. Furthermore, the NPCs are attached randomly within the confines of the lamin network, presumably by their nucleoplasmic rings. Image analysis of edge-on and en face projections of detergent-extracted NPCs has been combined with data on the coaxial thin rings to provide a quantitative evaluation of the triple ring model of NPC architecture proposed previously (Unwin, P. N. T., and R. Milligan. 1982. J. Cell Biol. 93:63-75). Additional details of the complex have been visualized including an intimate association of the inner spoke domains as an inner spoke ring, extensive domains within the spokes and coaxial thin rings, and interestingly, a central channel-like feature. Membrane-associated NPCs and detergent-extracted NPCs both possess peripherally located radial arms resulting in an effective diameter of approximately 1,450-1,500 A. In projection, the radial arms possess approximate mirror symmetry suggesting that they originate from both sides of the assembly. Moreover, membrane-associated NPCs are asymmetric at most radii and right-handed as viewed from the cytoplasm; detergent-extracted NPCs appear to be symmetric and have approximately 822 symmetry. Taken together, the data suggests that the framework of membrane-associated NPCs is perturbed from a symmetrical configuration, either during isolation of nuclei or by interactions with the lamina and the nuclear envelope in vivo. However, detergent extraction of nuclei appears to result in a more symmetrical alignment of components in apposing halves of the assembly.

A new monoclonal antibody recognizing the amino-terminal consensus sequence of vertebrate intermediate filament proteins

The mouse monoclonal antibody ME 101 raised against human peripherin, an intermediate filament protein (IFP) specific to well defined neuronal populations, recognizes all the major classes of vertebrate IFP in immunoblotting assays. Desmin, GFAP, vimentin, peripherin and the lightest neurofilament protein (NF-L) were cleaved into carboxy- and amino-terminal halves by N-chlorosuccinimide at their unique trytophan residue. Whereas the antibody directed against the epitope common to every IFP (intermediate filament antigen or IFA) and located on the carboxy-terminal end of the rod domain recognizes the carboxy-terminal half, the ME 101 antibody, as the present study illustrates, recognizes specifically the amino-terminal half. From the amino acid sequence data of IFP, it is deduced that the cognate epitope is localized on the amino-terminal part of coil la.

Lamin A, lamin B, and lamin B receptor analogues in yeast

Previous studies have shown that turkey erythrocyte lamin B is anchored to the nuclear envelope via a 58-kD integral membrane protein termed p58 or lamin B receptor (Worman H. J., J. Yuan, G. Blobel, and S. D. Georgatos. 1988. Proc. Natl. Acad. Sci. USA. 85:8531-8534). We now identify a p58 analogue in the yeast Saccharomyces cerevisiae. Turkey erythrocyte lamin B binds to yeast urea-extracted nuclear envelopes with high affinity, associating predominantly with a 58-kD polypeptide. This yeast polypeptide is recognized by polyclonal antibodies against turkey p58, partitions entirely with the nuclear fraction, remains membrane bound after urea extraction of the nuclear envelopes, and is structurally similar to turkey p58 by peptide mapping criteria. Using polyclonal antibodies against turkey erythrocyte lamins A and B, we also identify two yeast lamin forms. The yeast lamin B analogue has a molecular mass of 66 kD and is structurally related to erythrocyte lamin B. Moreover, the yeast lamin B analogue partitions exclusively with the nuclear envelope fraction, is quantitatively removed from the envelopes by urea extraction, and binds to turkey lamin A and vimentin. As many higher eukaryotic lamin B forms, the yeast analogue is chemically heterogeneous comprising two serologically related species with different charge characteristics. Antibodies against turkey lamin A detect a 74-kD yeast protein, slightly larger than the turkey lamin A. It is more abundant than the yeast lamin B analogue and partitions between a soluble cytoplasmic fraction and a nuclear envelope fraction. The yeast lamin A analogue can be extracted from the nuclear envelope by urea, shows structural similarity to turkey and rat lamin A, and binds to isolated turkey lamin B. These data indicate that analogues of typical nuclear lamina components (lamins A and B, as well as lamin B receptor) are present in yeast and behave as their vertebrate counterparts.

Nucleolin from Xenopus laevis: cDNA cloning and expression during development

Nucleolin is a key nucleolar protein in higher eukaryotic cells and is involved directly in ribosome biogenesis. Using an antiserum raised against hamster nucleolin, the homologous protein was detected in nucleoli of Xenopus laevis hepatocytes as well as in the amplified nucleoli of oocytes. A cDNA encoding Xenopus nucleolin has been isolated and sequenced. The deduced protein sequence reveals similar domains in Xenopus and in mammals, but they have undergone separate evolutions. In particular, each of the four RNA-binding domains has evolved differently--the carboxy-proximal domain is twice as conserved (87%) as the amino-proximal domain (42%). These data shed some light on the possible roles of each domain. The expression of nucleolin has been followed throughout oogenesis and embryogenesis. The appearance of nucleolin during early development precedes the transcription of rDNA and the synthesis of ribosomal proteins. The maximal accumulation of nucleolin at gastrulation coincides with nucleolar reformation. Furthermore, when ribosomal synthesis is activated during oogenesis and embryogenesis, peptides immunorelated to nucleolin appear and accumulate. The results suggest that nucleolin plays a role not only in ribosome assembly but also in nucleologenesis.

Interconversion of Drosophila nuclear lamin isoforms during oogenesis, early embryogenesis, and upon entry of cultured cells into mitosis

Two isoforms of a single nuclear lamin, distinguishable on one-dimensional SDS-polyacrylamide gels, have previously been identified in Drosophila nuclei during interphase. A third species, designated lamin Dmmit, has now been identified as soluble in extracts of Drosophila tissue culture cells blocked in mitosis by drugs. An apparently identical form is the only lamin species detectable in late-stage egg chambers and early embryos. Phosphoamino acid analyses suggest that the conversion of lamins Dm1 and Dm2 to lamin Dmmit is brought about by a specific rearrangement of phosphate groups rather than by dramatic net changes in the levels of lamin phosphorylation. The residues involved in these phosphorylation/dephosphorylation reactions have been tentatively mapped to a 17.8-kD cyanogen bromide fragment containing amino acids 385-547. This represents a potential "hinge" domain in the lamin structure between the end of coil 2 and the globular COOH terminus. These results have implications for understanding the regulation of nuclear envelope breakdown during mitosis and karyoskeletal dynamics during oogenesis and early embryogenesis.

Mitoskelin: a mitochondrial protein found in cytoskeletal preparations

A 70 kD protein, which we have named mitoskelin, is highly enriched in cytoskeletal preparations from bovine cardiac muscle. Mitoskelin has three main variants with isoelectric points between 5.6 and 5.8. Immunoblotting with polyclonal antibodies directed against mitoskelin shows that, like intermediate filament proteins, the majority of mitoskelin resists solubilization from a myocardial homogenate by a series of extraction solutions ranging from very low salt to 0.6 M KI buffers and by 0.1-1% Nonidet P-40 detergent. By double-label immunofluorescence on cells and tissues, mitoskelin is colocalized with the mitochondrial marker cytochrome c oxidase. Mitoskelin is associated with the inner membranes of mitochondria as shown by immunoelectron microscopy and immunoblotting. Immunological cross-reactivity and similarities of molecular weight, pI, distribution, and chromatographic properties indicate that mitoskelin is the 70 kD component of complex I (NADH: ubiquinone oxidoreductase), a portion of the mitochondrial oxidative phosphorylation system. No function or activity has yet been demonstrated for the 70 kD component of the 25-polypeptide complex I. Dialysis against physiological buffers allows purified, urea-solubilized mitoskelin to form 10 nm wide filamentous structures that do not closely resemble intermediate filaments. These results suggest the exciting possibility that mitochondria may contain a membrane-associated filamentous skeleton.

Nuclear protein transport

The nucleus, like all organelles, is composed of a unique set of proteins. This article discusses the possible mechanisms for localization of only certain proteins to the nucleus, transport of proteins across the nuclear envelope, and retention of proteins in the nuclear interior. In addition, nuclear protein transport is compared with transport of proteins into the endoplasmic reticulum and the mitochondria.

Large scale co-isolation of vimentin and nuclear lamins from ehrlich ascites tumor cells cultured in vitro

Ehrlich ascites tumor (EAT) cells propagated in mass suspension culture were used as a starting material for the simultaneous isolation and purification of large quantities of the intermediate filament protein vimentin and the nuclear lamins A/C and B. Triton cytoskeletons, obtained by repeated washing of cells with a low ionic strength buffer containing Triton X-100 and 4 mM Mg2+, were extracted with 6 M urea at low salt concentration and in the presence of EDTA. Separation of solubilized proteins from unfolded chromatin (DNA) was accomplished by recondensation of the chromatin (DNA) in the presence of Mg2+ before centrifugation. To achieve separation of vimentin from nuclear lamins, the urea extract was subjected to DEAE-Sepharose CL-6B chromatography. Single-stranded DNA-cellulose chromatography was employed for the final purification of vimentin and for the separation of lamin B from lamins A/C. Further purification of lamin B was carried out by CM-Sepharose CL-6B chromatography and of lamins A/C by chromatography on hydroxylapatite. All chromatographies were performed in the presence of 6 M urea. 500 g of pelleted EAT cells yielded approximately 700 mg of vimentin, 225 mg of lamins A/C and 21 mg of lamin B. 2D-polyacrylamide gel electrophoresis revealed great microheterogeneity of lamins A/C, which to a high extent was due to phosphorylation, whereas lamin B was much less heterogeneous. In the absence of urea and at low salt concentration, lamins A/C required pH 5 to stay in solution whereas lamin B required pH 7.5. Increasing the salt concentration to 150 or 250 mM NaCl resulted in the formation of paracrystals from a urea-free mixture of lamins A/C and B. Although the lamins could not be assembled into intermediate filaments under a variety of ionic conditions, the preparations obtained will be useful for further biochemical characterization of these nuclear proteins.

A lamin B receptor in the nuclear envelope

Using a solution binding assay, we show that purified 125I-labeled lamin B binds in a saturable and specific fashion to lamin-depleted avian erythrocyte nuclear membranes with a Kd of approximately 0.2 microM. This binding is significantly greater than the binding of 125I-labeled lamin A and is competitively inhibited by unlabeled ligand. We demonstrate that a 58-kDa integral membrane protein (p58) is a lamin B receptor by virtue of its abundance in the nuclear envelope and association with 125I-labeled lamin B in ligand blotting assays. Specific antibodies raised against p58 recognize one protein in isolated nuclei and partially block 125I-labeled lamin B binding to lamin-depleted nuclear membranes. Cell fractionation and indirect immunofluorescence microscopy show that p58 is located in the periphery of the nucleus. This protein may serve as a membrane attachment site for the nuclear lamina by acting as a specific receptor for lamin B.

Partial sequence of MAP2 in the region of a shared epitope with Alzheimer neurofibrillary tangles

A 3.3-kilobase DNA complementary to human microtubule-associated protein 2 (MAP2) was sequenced by the dideoxy method. The 3' end terminates at an internal EcoRI site before the polyA tail. Due to the arrangement of the cDNA insert in the lambda gt11 vector, the MAP2 fragment is not fused to beta-galactosidase when expressed. The Chou Fasman algorithm for the initial 58 amino acids from the first in-frame methionine predicts an alpha helix. Beyond this point, a series of turns is predicted until amino acid 160. The frequent presence of basic residues in proximity to serines or threonines is consistent with multiple phosphorylation sites. The minimum specificity determinant for Ca2+/calmodulin-dependent kinase is repeated 13 times. The sequence of a region containing a MAP2 epitope that is shared with the Alzheimer neurofibrillary tangle was determined by DNase treatment of the cDNA and antibody selecting the small resultant clones in a lambda gt11 sublibrary. Likewise, a MAP2 epitope that is not shared with the neurofibrillary tangle also has been located. Both epitopes are in the projection portion of the molecule. A bovine MAP2 cyanogen bromide fragment, which contains the epitope shared with the neurofibrillary tangle, is partially insoluble under aqueous conditions, probably due to the aggregation of oppositely charged residues. Thus, rapid cleavage of MAP2 to small peptides is probably necessary in vivo to prevent the aggregation of larger cleavage fragments.